1. Field of the Invention
The present invention relates to a system and method for controlling the injection of ammonia in a dry type exhaust gas denitration process.
2. Description of the Prior Art
In general, among the conventional dry type of exhaust gas denitration processes, the most predominant process is the catalytic denitration process in which nitrogen oxides (hereinafter represented by NO.sub.x) contained in a combustion exhaust gas and ammonia (hereinafter represented by NH.sub.3) injected from outside of the system and mixed with the exhaust gas make a catalytic reaction with an activator located on a catalyzer surface; the catalytic reaction decomposes the gas and ammonia into harmless nitrogen and water. Also, a non-catalytic denitration process in which NO.sub.x and NH.sub.3 make a vapor phase reaction at a high temperature region so as to be decomposed, is being developed up to the stage of practical utilization.
On the other hand, most of NO.sub.x contained in a combustion exhaust gas is actually NO, and the proportion of NO to NO.sub.x will vary somewhat depending upon temperature conditions such as combustion temperature or the like. However, in the denitration processes described above, an equivalent reaction between NO and NH.sub.3, that is, a reaction of: EQU 4NO+4NH.sub.3 +O.sub.2 .fwdarw.4N.sub.2 +6H.sub.2 O
is deemed to be the principal reaction, and so, NO.sub.x is decomposed and removed while injecting, under automatic control, NH.sub.3 proportional to the NO.sub.x equivalent to be decomposed or to a somewhat larger amount of NO.sub.x.
In the prior art method for controlling the injection of NH.sub.3, the amount of NO.sub.x in a combustion exhaust gas was detected by multiplying the NO.sub.x concentration by the combustion exhaust gas flow rate, and the amount of injection of NH.sub.3 was set by multiplying the detected amount of NO.sub.x by the intended NH.sub.3 /NO.sub.x ratio. Since the NH.sub.3 /NO.sub.x ratio was manually set or fixedly set, the ratio was kept constant. However, the above-described prior art method for controlling the injection of NH.sub.3 had the following shortcomings:
(1) Though the NO.sub.x concentration is detected by means of an automatic analyzer, before the results of the measurement are converted into transmission signals and then outputted, a delay of one minute or more exists due to the delays inherent in the analyzer system, (for example, a delay caused by the replacement of gas in a sampling line), so that a considerable delay occurs in the control of the NH.sub.3 injection amount, and accordingly, the proper denitration operation cannot follow any abrupt system parameter changes. PA1 (2) The denitration reaction speed between NO.sub.x and NH.sub.3 will vary depending upon temperature, and also the temperature dependency of the denitration operation will vary depending upon the kind of catalyzer used, so that in case where the NH.sub.3 /NO.sub.x ratio is constant, the optimum denitration rate cannot be always obtained. PA1 (3) When the combustion exhaust gas temperature becomes low (for instance, 300.degree. C. or lower), a poisoning effect for the catalyzer will arise due to absorption of NH.sub.3 onto the catalyzer surface and due to the ammonium salt formed by the reaction between SO.sub.x in the combustion exhaust gas and NH.sub.3, so that it is necessary to lower the NH.sub.3 /NO.sub.x ratio, and in prior art methods, it was sometimes necessary to stop the injection of NH.sub.3 at low exhaust gas temperatures, interrupting the denitration process. PA1 (4) In addition, as the temperature is lowered, the amount of absorption of NH.sub.3 onto the catalyzer surface is increased, and upon a temperature rise, the NH.sub.3 absorbed during the low temperature is released and dispersed in the gas, so that the NH.sub.3 /NO.sub.x ratio in the gas is raised, resulting in the phenomenon that NH.sub.3 is released with the gas at the outlet of the denitration apparatus. Therefore, not only is there an adverse effect on associated instruments succeeding the outlet of the denitration apparatus, (for example, an air heater), but also the released NH.sub.3 may possibly become a public nuisance. PA1 (1) use is made of at least a first processor unit in which on the basis of the relationship between the produced NO.sub.x concentration and the numerical value representing an amount of combustion, such as the fuel flow rate, the combustion air flow rate, the combustion exhaust gas flow rate, the water supply flow rate or the generated vapor flow rate, derives and stores the amount of NO.sub.x as a function of said numerical value representing the amount of combustion, and use is made of a second processor unit in which on the basis of the relationship between the combustion exhaust gas temperature and the denitration performance, derives and stores the NH.sub.3 /NO.sub.x ratio as a function of said combustion exhaust gas temperature; the amount of NO.sub.x obtained by inputting said numerical value representing the amount of combustion to said first processor unit is multiplied by the NH.sub.3 /NO.sub.x ratio obtained by inputting the combustion exhaust gas temperature or the numerical value corresponding to the combustion exhaust gas temperature to said second processor unit; PA1 (2) use is made of at least a first processor unit, a second processor unit and a third processor unit in which on the basis of the relationship between the time variation rate of the combustion exhaust gas temperature and the NH.sub.3 /NO.sub.x ratio, a characteristic factor for correcting the NH.sub.3 /NO.sub.x ratio as a function of said time variation rate is derived and stored; the amount of NO.sub.x is obtained by inputting said numerical value representing the amount of combustion to said first processor unit is multiplied by the NH.sub.3 /NO.sub.x ratio obtained by inputting the combustion exhaust gas temperature or the numerical value corresponding to the combustion exhaust gas temperature to said second processor unit and correcting the NO.sub.x amount by means of said third process unit.